The reliability of toxic gas detectors is of great importance in many applications, especially when these instruments are used for ensuring the safety of personnel. Reliability is typically obtained by periodic checking of the instrument response to a test gas, however calibration test gases are typically supplied in large, bulky and expensive gas cylinders.
Potentially hazardous atmospheres are found in many locations, due to the presence of toxic gases, combustible gas mixtures or the excess or deficiency of oxygen concentration. Many types of gas detection instruments have been developed to provide a warning that the atmosphere contains potentially hazardous components, or to initiate remedial action. Examples of these gas detection instruments include the detection of combustible gases in coal mines, hydrogen sulfide in oil fields and water treatment plants, carbon monoxide in places ranging from steel mills to bedrooms, and oxygen in confined spaces, such as sewers. Within each gas detection instrument there are one or more gas sensors, whose function is to provide an electrical signal, which varies in response to the gas concentration.
Most gas sensors provide a relative output signal, such that the output signal is not an absolute measure of gas concentration, but merely proportional to the gas concentration. In such cases, the gas sensor must be calibrated with a known test gas prior to use. Calibration can also be used as a function check to ensure the sensor is working. The output from many types of sensors can vary over time and sensors can fail to operate without warning. Frequently calibrating a gas sensor can be time consuming, expensive and cumbersome in many applications. Calibrating a gas sensor is also limited to the reproducibility of the amount of test gas in contact with the sensor.